Heat Exchanger for Heating a Fluid Using Exhaust Gas

ABSTRACT

A rapid warm-up heat exchanger for heating a fluid using exhaust gas includes multiple plate pairs that are joined by braze joints to form a stack. A fluid inlet manifold and a fluid manifold extend through the stack, and each one of the plate pairs defines a tortuous flow path for the fluid that extends between the fluid inlet and fluid outlet manifolds. A housing surrounds the stack, and together the housing and the stack define an exhaust flow path in spaces provided between adjacent plate pairs. A valve element can be provided within the housing in order to selectively direct exhaust flow through the exhaust flow path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/338,695, filed May 19, 2016, the entire contents of which are hereby incorporated herein by reference.

BACKGROUND

Heat exchangers to transfer heat from a flow of exhaust gas to coolant are well known. Such heat exchangers typically are used in exhaust gas recirculation (EGR) systems for internal combustion engines, wherein exhaust gas from the engine is recirculated back to the intake manifold of the engine in order to reduce harmful and undesirable emissions, partially oxides of nitrogen. In such systems, the heat exchanger or EGR cooler is used to reduce the temperature of the exhaust gas in order to produce more favorable conditions for limiting the production of these emissions. Heat exchangers of this type are typically constructed of stainless steel alloys, with exhaust gas being conveyed through tube structures while liquid coolant washes over the outer surfaces of the tube structures.

In addition to reducing the harmful and undesirable pollutant emissions resulting from combustion, much emphasis is placed upon improving the overall drive cycle efficiency of internal combustion engine powered vehicles. This efficiency can be substantially degraded by the vehicle's operation at cold-start conditions, before the engine and associated powertrain systems have reached their normal operating temperatures. In order to maximize the overall drive cycle efficiency, it thus becomes desirable for the engine and powertrain system to warm up to normal operating temperatures as rapidly as possible.

Such rapid warm-up can be achieved through recovery of otherwise wasted heat energy from the exhaust produced by the combustion engine, which exits the combustion chambers at a relatively high temperature even before the remainder of the vehicular systems have achieved any significant warming. Effectively transferring heat energy from the exhaust gas flow to a flow of engine coolant in order to heat that coolant so that it can subsequently warm up other parts of the engine and powertrain system (for example, the transmission, through heat transfer from the warmed coolant to transmission oil) is an especially desirable way to achieve this rapid warm-up.

Existing EGR coolers, while suitable for transferring heat energy from exhaust gas to a coolant, are not particularly well-suited to achieve the aforementioned rapid warm-up. Such heat exchangers are primarily designed to allow for large flow rates of coolant to pass through the heat exchanger in order to effectively cool the exhaust gas with minimal heating of the coolant. In contrast, rapid warm-up requires a more limited flow of fluid so that the temperature rise of the fluid as it passes through the heat exchanger can be maximized. The primary goal of a rapid warm-up heat exchanger is to heat the fluid using the exhaust gas as a heat source, rather than to cool the exhaust gas using the fluid as a heat sink. Thus, there is still room for improvement.

SUMMARY

According to an embodiment of the invention, a heat exchanger for heating a fluid using exhaust gas includes multiple plate pairs that are joined by braze joints to form a stack. A fluid inlet manifold and a fluid manifold extend through the stack, and each one of the plate pairs defines a tortuous flow path for the fluid that extends between the fluid inlet and fluid outlet manifolds. A housing surrounds the stack, and together the housing and the stack define a generally U-shaped exhaust flow path in spaces provided between adjacent plate pairs.

In some embodiments, the generally U-shaped exhaust flow path surrounds the inlet and outlet fluid manifolds. In some embodiments turbulation features extend from the plates into the spaces between adjacent plate pairs in order to turbulate the exhaust gas flow.

In some embodiments, the housing includes at least two parts. One or more of the housing parts can be joined to the stack by braze joints. In some such embodiments the braze joints between the plate pairs and the braze joints joining one or more of the housing parts to the stack are formed in a single brazing operation. Two of the housing parts can, in some embodiments, be joined together in a plane that is parallel to, and located between, a topmost plate of the stack of plate pairs and a bottommost plate of the stack of plate pairs.

In some embodiments the housing includes a wall that is arranged adjacent to an edge of each of the plate pairs. The wall includes an inlet aperture to deliver exhaust gas to the exhaust flow path, and an outlet aperture spaces apart from the inlet aperture to receive exhaust gas from the exhaust flow path. The wall further includes a wall section extending between the inlet aperture and the outlet aperture. The fluid inlet and outlet manifolds are arranged immediately adjacent to that wall section. In some such embodiments the housing includes a first housing part containing the inlet aperture, and a separate second housing part containing the outlet aperture. The wall section between the inlet and outlet apertures can be defined by portions of both the first and the second housing parts.

In some embodiments, the housing includes a cylindrical exhaust inlet, a cylindrical exhaust outlet, and a linear flow path extending between the cylindrical exhaust inlet and the cylindrical exhaust outlet. The linear flow path bypasses the generally U-shaped exhaust flow path. In some embodiments the heat exchanger includes a valve element that is arranged along the linear flow path to selectively divert exhaust gas along the generally U-shaped exhaust flow path.

In some embodiments, The housing can include multiple parts, with a first portion of each of the cylindrical inlet and the cylindrical outlet being defined by one housing part and with a second portion of each of the cylindrical inlet and the cylindrical outlet being defined by another housing parts. A third portion of the cylindrical exhaust inlet and the cylindrical exhaust outlet can be defined by a third housing part, which can be joined to the other two housing parts in a plane that is parallel to a stacking direction of the plate pairs.

According to another embodiment of the invention, a heat exchanger for heating a fluid using exhaust gas includes multiple plate pairs that are joined by braze joints to form a stack, with spaces between adjacent plate pairs at least partially defining a flow path for the exhaust gas. A housing that includes at least a first and a second housing part surrounds the stack of plate pairs in order to bound the exhaust gas within the heat exchanger. At least one of the housing parts is joined to the stack by braze joints. A valve element is arranged within the housing to selectively divert exhaust gas so that it bypasses the spaces between the plate pairs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heat exchanger according to an embodiment of the present invention.

FIG. 2 is a sectioned plan view of the heat exchanger of FIG. 1.

FIG. 3 is a perspective view of a stack of plate pairs for use in the heat exchangers of FIGS. 1 and 7.

FIG. 4 is an elevation view of the stack of plate pairs of FIG. 3.

FIG. 5 is an exploded perspective view of a single plate pair of the stack of FIG. 3.

FIG. 6 is a plan view of one of the plates of FIG. 5.

FIG. 7 is a perspective view of a heat exchanger according to another embodiment of the invention.

FIG. 8 is a sectioned plan view of the heat exchanger of FIG. 7.

FIG. 9 is an exploded perspective view of the heat exchanger of FIG. 7.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

A heat exchanger 1 according to an embodiment of the present invention is depicted in FIGS. 1-2, and includes a stack 2 of plate pairs contained within a housing 10. The stack 2 and the housing 10 are preferably constructed of metal alloys, and are preferably joined to form the heat exchanger 1 through a brazing operation. In some especially preferable embodiments the metal alloys used to construct the components of the heat exchanger 1 are steel alloys such as, for example, austenitic or ferritic stainless steels. In other embodiments at least some of the components can be constructed of other alloys, including aluminum alloys, copper alloys, and the like. Braze alloy material to create the braze joints between the parts can be provided as a clad layer on some or all of the parts, as a spray or paste, as a foil interleaved between parts, or by other known methods.

The housing 10 is, in the exemplary embodiment of FIGS. 1-2, constructed from several stamped metal parts that surround the stack 2 and engage one another to form an enclosure. A first housing part 12 and a second housing part 13 are both generally U-shaped stampings that together surround the sides of the stack 2. One or both of the housing parts 12, 13 can be provided with stiffening beads 40 formed into the part, as shown in FIG. 1. An overlapping joint 5 between the housing parts 12, 13 is arranged approximately midway long two opposing sides of the stack 2, and the housing parts 12, 13 are preferably joined to one another by braze joints at those overlapping joint locations 5. A housing top plate is provided at one end of the stack 2, and a housing bottom plate 42 is provided at the opposing end of the stack 2. The housing top plate 41 and the housing bottom plate 42 are both also preferably provided as stamped metal pieces. A peripheral flange 47 is provided along the edges of both plates 41 and 42, and overlaps the edges of the housing parts 12 and 13. The housing top plate 41 and the housing bottom plate 42 are preferably joined to the housing parts 12 and 13 by braze joints at the overlapping flanges 47.

An aperture 16 is provided on the housing part 12, and functions as an inlet for a flow of exhaust gas that is directed into the heat exchanger 1. Similarly, an aperture 17 is provided on the housing part 13 and functions as an outlet for the flow of exhaust gas. Each of the apertures 16, 17 is provided with a peripheral flange 48 along its perimeter, enabling the connection of the heat exchanger 1 to an exhaust system in order to provide exhaust gas to the heat exchanger 1 from upstream sections of the exhaust system, and to provide the exhaust gas from the heat exchanger 1 to downstream sections of the exhaust system. While the apertures 16, 17 are depicted as having a rectangular shape with rounded corners, it should be understood that the shapes of the apertures 16, 17 can vary with the needs of the exhaust system in which the heat exchanger 1 is used.

Turning now to FIGS. 3-6, the stack 2 of plate pairs will be described in further detail. As best seen in FIG. 5, each of the plate pairs 3 includes a plate 4 a and a plate 4 b that are joined together to form the plate pair 3. The plates 4 a and 4 b are stamped plates incorporating a variety of features to provide both a flow path 8 for a fluid passing through the plate pair 3, and a flow path 9 for exhaust gas passing between adjacent plate pairs 3. The plates 4 a and 4 b both include dished central regions that face one another when the plates 4 a, 4 b are joined along their peripheries by braze joints to form the plate pair 3, thereby providing the flow space for the fluid. Internally facing dimples 15 are provided at select locations along the surface of the plate 4 b. Those dimples 15 are disposed against the inwardly facing surface of the plate 4 a when the plate pair 3 is assembled, and braze joints are developed at those locations during brazing of the stack 2. Those braze joints provide enhanced structural support for the plate pair 3, especially when subjected to elevated internal pressurization. In addition, the dimples 15 are staggered to provide flow interruptions for the fluid passing through the plate pair 3, thereby turbulating the flow of the fluid and providing enhanced heat transfer.

At least one inwardly facing elongated bead feature 38 is additionally provided on the plate 4 b and brazed to the plate 4 a. In addition to providing enhanced structural support for the plate pair 3, the elongated bead feature 38 also defines a tortuous flow path 8 for the fluid. The tortuous flow path 8 extends between an inlet manifold 6 and an outlet manifold 7 extending through the stack 2. A pair of circular apertures 36 is provided in each of the plates 4 a and 4 b, and corresponding ones of the apertures 36 are aligned to define the manifolds 6, 7. Outwardly facing peripheral flanges 37 surround each of the apertures 36, and engage with the flanges 37 of the plates of adjacent plate pairs in order to define continuous and leak-free fluid manifolds when the stack 2 is brazed.

The manifolds 6 and 7 are arranged to be adjacent one another and separated by a portion of the elongated bead feature 38 in order to prevent the direct flow of fluid between the inlet manifold 6 and the outlet manifold 7 without first passing through the remainder of the space within the plate pair 3. The manifolds 6, 7 are generally centrally located along a length direction (indicated by arrow 49) of the plate pair 3, and are disposed along a common edge of each of the plurality of plate pairs 3 in the width direction (indicated by arrow 50) of the plate pair 3. The fluid is directed into the internal chamber of the plate pair 3 from the inlet manifold 6 and is directed by the elongated bead feature 38 to flow in the lengthwise direction 49 away from the center of the plate pair 3. The fluid flow is subsequently directed, through a series of right-angle turns, to flow along the tortuous flow path 8, with alternating portions of the flow path 8 being generally aligned with the width direction 50 and the length direction 49. In the exemplary embodiment, as best seen in FIG. 6, the tortuous flow path 8 includes nine such alternating portions.

The plates 4 a and 4 b are additionally provided with outwardly facing dimples 32 and elongated beads 31. The elongated beads 31 are aligned so that they extend in a longitudinal direction that is arranged at a non-perpendicular angle to both the lengthwise direction 49 and the widthwise direction 50. Preferably, the elongated beads 31 are at angles ranging from 30° to 60° with respect to both the lengthwise direction 49 and the widthwise direction 50. A first group of the elongated beads 31 on each of the plates 4 a, 4 b have their longitudinal directions aligned along a first direction, and a second group of the elongated beads 31 on each of the plates 4 a, 4 b have their longitudinal directions aligned along a second direction, so that the second group of elongated beads 31 are mirror-image reflections of the first group of elongated beads 31 in both the lengthwise direction 49 and the widthwise direction 50. Approximately half of the elongated beads 31 of each of the plates are members of the first group, and approximately half are members of the second group.

The elongated beads 31 are generally arranged in rows and columns, with the rows extending in the lengthwise direction 49 and the columns extending in the widthwise direction 50. Along each of the rows and the columns the elongated beads 31 are alternatingly members of the first group and the second group. Elongated beads 31 are avoided in those areas that correspond to the locations of the inwardly facing elongated beat features 38. In addition, the elongated beads 31 are not provided at locations where the outwardly facing dimples 32 are located.

Additional outwardly facing bead features 39 are provided on the plates 4 b directly adjacent to the circular apertures 36. Similar features can be provided in corresponding location on the plates 4 a. Alternatively, the outwardly facing dimples 32 can be provided on the plate 4 a in corresponding locations, as shown in the exemplary plate 4 a of FIG. 5.

At least some of the outwardly facing dimples 32 and/or elongated beads 31 and/or bead features 39 of adjacent plate pairs 3 are in contact with one another when the plate pairs 3 are arranged into the stack 2. Spaces 11 are thereby created between the planar surfaces of the plates in adjacent plate pairs. In the exemplary embodiment, the heights of the dimples 32 and the bead features 39 are slightly greater than the heights of the beads 31, so that the beads 31 of adjacent plates are not in contact with one another. When the plate pairs 3 are assembled into the stack 2, each of the beads 31 of a plate 4 a belonging to the first group is disposed adjacent to a bead 31 of a plate 4 b belonging to the second group, and vice-versa. In other words, the beads 31 within a space 11 are arranged in mirror-image pairs. Such an arrangement provides for increased turbulation of the exhaust flow passing through the spaces 11, thereby improving the rate of heat transfer.

In the exemplary embodiment of FIGS. 1-2, the exhaust gas inlet and outlet apertures 16, 17 are both provided within a common wall 49 of the housing. The wall 49 is located directly adjacent to that edge of each of the plate pairs 3 that the fluid manifolds 6 and 7 are disposed along. A wall section 18 of the wall 49 is provided between the inlet aperture 16 and the outlet aperture 17, with one part of that wall section 18 being provided by the first housing part 12 and another part of that wall section 18 being provided by the second housing part 13. The fluid inlet manifold 6 and the fluid outlet manifold 7 are arranged immediately adjacent the wall section 18. The spaces 11 between adjacent plate pairs 3 thereby define a generally U-shaped exhaust gas flow path 9 extending from the inlet aperture 16 to the outlet aperture 17. The exhaust flow path 9 thereby extends around the fluid manifolds 6, 7, passing over the beads 31.

As the exhaust gas passes through the heat exchanger 1 along the exhaust gas flow path 9, and the fluid to be heated passes through the heat exchanger 1 along the tortuous fluid flow path 8, heat is transferred through the plates 4 from the exhaust gas to the fluid. The rate of such heat transfer can be maximized by having the fluid inlet manifold 6 arranged closest to the exhaust outlet aperture 17 and by having the fluid outlet manifold arranged closest to the exhaust inlet aperture 16, as reflected in the exemplary embodiment of the heat exchanger 1. Such an arrangement provides that the exhaust gas and the fluid to be heated pass through the heat exchanger 1 in a counterflow arrangement, with the coldest fluid (i.e. the fluid immediately after being received into the plate pairs 3 from the inlet manifold 6) receiving heat from the coldest exhaust gas (i.e. the exhaust gas immediately prior to being removed through the exhaust outlet aperture 17) and with the hottest fluid (i.e. the fluid immediately prior to being removed from the plate pairs 3 into the outlet manifold 7) receiving heat from the hottest exhaust gas (i.e. the exhaust gas immediately after being received into the spaces 11 from the exhaust inlet aperture 16). In some alternative embodiments, it may be equally desirable or more desirable to reverse the flow of either the fluid to be heated or the exhaust gas.

A fluid fitting block 33 is disposed on one end of the stack 2, and is provided with an inlet port 34 and an outlet port 35 in direct fluid communication with the manifolds 6 and 7, respectively. The ports 34 and 35 enable an easy fluid connection of the heat exchanger 1 into a fluid circuit such as, for example, an engine coolant circuit, so that the fluid to be heated by the exhaust gas can be circulated through the heat exchanger 1. A complementary embossment feature 46 is provided on the top plate 41 of the housing 10 to receive the fitting block 33 and to allow the ports 34 and 35 to be accessed.

The various components can advantageously be assembled and joined together to form the monolithic heat exchanger 1 through a single brazing operation. The plate pairs 3 are first stacked to form the stack 2, along with the fitting block 33 and the ports 34, 35. After the stack 2 is assembled, the first and second housing parts 12, 13 are assembled around the stack 2 from opposing sides. The housing top plate 41 and the housing bottom plate 42 are next assembled, capturing the edges of the housing parts 12, 13 to fix their position. The assembled heat exchanger 1 can then be heated in a brazing furnace to a temperature that is suitable for melting the braze alloy, causing joints to form between the adjacent components.

In another embodiment of the invention, illustrated in FIGS. 7-9, the stack 2 is incorporated into a housing 110 along with a valve element 22 to form a heat exchanger 101. The heat exchanger 101 is again well suited to transfer heat from a flow of exhaust gas to a fluid in order to heat the fluid, but also allows the exhaust gas to pass through the heat exchanger 101 without heating the fluid when such heating is undesirable. The housing 110 is provided with a cylindrical exhaust inlet 19 and a cylindrical exhaust outlet 20, with axes of the inlet 19 and outlet 20 aligned to facilitate integration of the heat exchanger 101 with a cylindrical exhaust pipe system.

The housing 110 is formed by a first housing part 112, a second housing part 113, and a third housing part 114. The first and second housing parts 112, 113 cooperate to encapsulate three of the sides and the top and bottom of the stack 2 when joined together. An embossment feature 146 to receive the fitting block 33 is provided on the housing part 112, in similar fashion to the embossment 46 of the previously described embodiment. An overlapping joint between the parts 112 and 113 allows for a self-fixturing assembly between the two parts, so that those parts are joined together in a plane 29 that is parallel to, and located between, a top-most plate of the plurality of plate pairs 3 that define the stack 2, and a bottom-most plate of the plurality of plate pairs 3. In the exemplary embodiment of FIG. 7 the plane 29 is located approximately midway along the stack height, but it should be understood that the plane 29 can be located anywhere along the height of the stack 2.

The valve element 22 can be variably positioned within the housing 110 in order to selectively direct the flow of exhaust gas through the spaces 11 of the stack 2, or entirely bypassing the stack 2, or in some combination of through the spaces 11 and bypassing. As best seen in FIG. 8, when the valve element 22 is positioned against a valve seat 45, all or substantially all of the exhaust gas entering the housing 110 through the cylindrical exhaust inlet 19 is directed through the U-shaped exhaust gas flow path 9 that extends through the spaces 11 of the stack 2. However, the position of the valve element can be adjusted to an alternative position, illustrated by the dashed line 22′. When the valve element 22 is in that alternative position, flow of exhaust gas through the stack 2 is substantially or completely prevented, and the exhaust gas instead flows along a linear flow path extending between the cylindrical exhaust gas inlet 19 and the cylindrical exhaust gas outlet 20, indicated by the dashed line flow path 21. Such an operation may be desirable when, for example, the engine producing the exhaust gas has not been operating for a long enough period of time to sufficiently heat the exhaust gas.

A hole 43 extends linearly through the first housing part 112 and the second housing part 113 to provide a pivot axis along which the valve element 22 can be rotated in order to vary the position of the valve element 22. A valve actuator (not shown) can be disposed in close proximity to the heat exchanger 101 and can include a shaft that extends through the hole 43 in order to control the position of the valve element 22. A formed part 44 can be incorporated within the housing 110 to assist in directing the flow of exhaust gas, and can be disposed against the stack 2 in order to prevent the bypassing of exhaust gas around the stack 2 when the valve element 22 is seated against the valve seat 45. The part 44 is preferably provided with an arcuate surface that is coaxial with the hole 43 so that the valve element 22 can seal against the part 44 to prevent bypass while remaining free to pivot about the axis of the hole 43.

The third housing part 114 is joined to the parts 112 and 113 at a plane 30 that is oriented perpendicularly to the plane 29 in the exemplary embodiment, although non-perpendicular arrangements of the planes 29, 30 are also contemplated. The plane 30 can be advantageously located so as to coincide with the location of the aligned axes of the cylindrical exhaust inlet 19 and outlet 20, so that the housing part 114 defines an approximately 180 degree arc length of each of the inlet 19 and outlet 20. The parts 112 and 113 together define the remaining 180 degrees of arc length of each of the inlet 19 and outlet 20, and each define an approximately 90 degree arc length of each of the inlet 19 and outlet 20 when the plane 29 is located approximately midway along the stack height. The cylindrical exhaust inlet 19 is thus defined, in the exemplary embodiment, by a 90 degree arc portion 23 provided by the first housing part 112, a 90 degree arc portion 24 provided by the housing part 113, and a 180 degree arc portion 25 provided by the housing part 114. Similarly, cylindrical exhaust outlet 20 is defined, in the exemplary embodiment, by a 90 degree arc portion 26 provided by the first housing part 112, a 90 degree arc portion 27 provided by the housing part 113, and a 180 degree arc portion 28 provided by the housing part 114. Joints between the parts 114 and the parts 112 and 113 can be created by overlapping of the material, allowing for a self-fixturing assembly of the housing part 114.

In a highly preferred embodiment, at least some of the parts of the housing 110 are joined together with the stack 2 in a common brazing operation. By way of example, the housing parts 112 and 113, along with the formed part 44, can be assembled to the stack of plate pairs 3 prior to brazing of the stack 2, in order that those parts are joined the stack 2 concurrently with the brazing of the stack 2. Subsequent to that brazing operation, the valve element 22 can be assembled in from the open end of the housing 110 generally corresponding to the plane 30. Additionally, the shaft that couples the vale element 22 to a valve actuator can be assembled at that time. Once assembly of the valve element 22 is completed, the remaining part 114 of the housing 110 can be assembled to the housing parts 112, 113 by, for example, welding.

Various alternatives to the certain features and elements of the present invention are described with reference to specific embodiments of the present invention. With the exception of features, elements, and manners of operation that are mutually exclusive of or are inconsistent with each embodiment described above, it should be noted that the alternative features, elements, and manners of operation described with reference to one particular embodiment are applicable to the other embodiments.

The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention. 

What is claimed is:
 1. A heat exchanger for heating a fluid using exhaust gas, comprising: a plurality of plate pairs joined by braze joints to form a stack; a fluid inlet manifold and a fluid outlet manifold extending through the stack, each one of the plate pairs defining a tortuous flow path for the fluid extending between the fluid inlet manifold and the fluid outlet manifold; and a housing surrounding the stack, the housing and the stack together defining a generally U-shaped exhaust flow path in spaces provided between adjacent plate pairs.
 2. The heat exchanger of claim 1, wherein the U-shaped exhaust flow path surrounds the inlet and outlet fluid manifolds.
 3. The heat exchanger of claim 1, wherein the housing comprises at least two parts and wherein one or more of the at least two parts is joined to the stack by braze joints.
 4. The heat exchanger of claim 3, wherein the braze joints between the pluralities of plate pairs and the braze joints joining the one or more of the housing parts to the stack are formed in a single brazing operation.
 5. The heat exchanger of claim 1, further comprising a plurality of turbulation features extending from plates of the plate pairs into the spaces between adjacent plate pairs.
 6. The heat exchanger of claim 1, wherein the housing includes a wall adjacent to an edge of each of the plurality of plate pairs, said wall comprising: an inlet aperture to deliver exhaust gas to the exhaust flow path; an outlet aperture spaced apart from the inlet aperture to receive exhaust gas from the exhaust flow path; and a wall section extending between the inlet aperture and the outlet aperture, the fluid inlet and outlet manifolds being arranged immediately adjacent the wall section.
 7. The heat exchanger of claim 6, wherein the housing comprises: a first housing part containing the inlet aperture; and a second housing part distinct from the first housing part, the second housing part containing the outlet aperture, wherein the wall section between the inlet aperture and the outlet aperture is defined by portions of both the first housing part and the second housing part.
 8. The heat exchanger of claim 1, wherein the housing includes: a cylindrical exhaust inlet; a cylindrical exhaust outlet; and a linear flow path extending between the cylindrical exhaust inlet and the cylindrical exhaust outlet, the linear flow path bypassing the generally U-shaped exhaust flow path.
 9. The heat exchanger of claim 8, further comprising a valve element arranged along the linear flow path to selectively direct exhaust gas along the generally U-shaped exhaust flow path.
 10. The heat exchanger of claim 9, wherein the valve element is rotatable about an axis extending through the housing.
 11. The heat exchanger of claim 7, wherein the housing comprises: a first housing part defining a first portion of the cylindrical exhaust inlet and a first portion of the cylindrical exhaust outlet; and a second housing part defining a second portion of the cylindrical exhaust inlet and a second portion of the cylindrical exhaust outlet.
 12. The heat exchanger of claim 11, wherein at least one of the first and second housing parts is joined to the stack by braze joints.
 13. The heat exchanger of claim 11, wherein both the first and second housing parts are joined to the stack by braze joints.
 14. The heat exchanger of claim 11, wherein the first and second housing parts are joined together in a plane that is parallel to, and located between, a top-most plate of the plurality of plate pairs and a bottom-most plate of the plurality of plate pairs.
 15. The heat exchanger of claim 11, wherein the housing further comprises a third housing part defining a third portion of the cylindrical exhaust inlet and a third portion of the cylindrical exhaust outlet.
 16. The heat exchanger of claim 15, wherein the third housing part is joined to both the first and second housing parts in a plane that is parallel to a stacking direction of the plurality of plate pairs.
 17. A heat exchanger for heating a fluid using exhaust gas, comprising: a plurality of plate pairs joined by braze joints to form a stack, spaces between adjacent plate pairs at least partially defining a flow path for the exhaust gas; a housing surrounding the plurality of plate pairs to bound the exhaust gas within the heat exchanger, the housing comprising a first housing part and a second housing part, at least one of the first and second housing parts being joined to the stack by braze joints; and a valve element arranged within the housing to selectively divert exhaust gas so as to bypass the spaces between the plurality of plate pairs.
 18. The heat exchanger of claim 17, wherein the housing includes a cylindrical exhaust inlet and a cylindrical exhaust outlet, a first portion of the cylindrical exhaust inlet and a first portion of the cylindrical exhaust outlet being defined by the first housing part and a second portion of the cylindrical exhaust inlet and a second portion of the cylindrical exhaust outlet being defined by the second housing part.
 19. The heat exchanger of claim 18, wherein the housing further comprises a third housing part defining a third portion of the cylindrical exhaust inlet and a third portion of the cylindrical exhaust outlet.
 20. The heat exchanger of claim 19, wherein the first and second portions of the cylindrical exhaust inlet and outlet each span an arc length of 90 degrees and wherein the third portions of the cylindrical exhaust inlet and outlet each span an arc length of 180 degrees. 